The objectives of the present review are to describe the agreement between variables on arterial and venous blood gas analysis (in particular pH, pCO(2) , bicarbonate and base excess) and to identify unanswered questions. MEDLINE search of papers published from 1966 to January 2010 for studies comparing arterial and peripheral venous blood gas values for any of pH, pCO(2) , bicarbonate and base excess in adult patients with any condition in an emergency department setting. The outcome of interest was mean difference weighted for study sample size with 95% limits of agreement. The weighted mean arterio-venous difference in pH was 0.035 pH units (n= 1252), with narrow limits of agreement. The weighted mean arterio-venous difference for pCO(2) was 5.7 mmHg (n= 760), but with 95% limits of agreement up to the order of ±20 mmHg. For bicarbonate, the weighted mean difference between arterial and venous values was -1.41 mmol/L (n= 905), with 95% limits of agreement of the order of ±5 mmol/L. Regarding base excess, the mean arterio-venous difference is 0.089 mmol/L (n= 103). There is insufficient data to determine if these relationships persist in shocked patients or those with mixed acid-base disorders. For patients who are not in shock, venous pH, bicarbonate and base excess have sufficient agreement to be clinically interchangeable for arterial values. Agreement between arterial and venous pCO(2) is too poor and unpredictable to be clinically useful as a one-off test but venous pCO(2) might be useful to screen for arterial hypercarbia or to monitor trends in pCO(2) for selected patients.